The pick off is a magnetic sensor such as a Hall effect or an MRE device.
The Hall device gives an electrical impulse every time a blade passes under the
sensor, whereas the resistance of the MRE device changes in a changing magnetic
field. Figure 12.14a shows the circuit used to shape the signal from an MRE
into a digital signal. The MRE sensor contains four elements to form a bridge
circuit as shown. The Hall or MRE device does not normally require temperature
compensation as they are being used as switches in digital configurations.
To measure flow rates a window is opened for a known time period. The number
of impulses from these devices are counted from which the rate of flow can be
calculated.